Power Management Device for a Micro Grid

Containing a PV System and a Generator

زيع طاقة صغيرة تحتوي على مولد تصميم جهاز إلدارة شبكة تو

مصدر للطاقة الشمسيةو

Ahmed Yousef Sokkar

Supervised by

Prof. Dr. Mohamed M. Abdelati

Prof. of Electrical Engineering

A thesis submitted in partial fulfilment

of the requirements for the degree of

Master of Electrical Engineering

November/2016

زةــغ – ةــالميــــــة اإلســـــــــامعـجال

شئون البحث العلمي والدراسات العليا

ةـــــــدســــــــــــــهنال ةـــــــــــــــليــكـ

ربائيةــــــة الكهــــــر الهندستيساجــم

االت ـــــــــــــــــة االتصــــــــــــــــأنظم

The Islamic University–Gaza

Research and Postgraduate Affairs

Faculty of Engineering

Master of Electrical Engineering

Communications systems

II

إقــــــــــــــرار

أنا الموقع أدناه مقدم الرسالة التي تحمل العنوان:

Power Management Device for a Micro Grid

Containing a PV System and a Generator

تصميم جهاز إلدارة شبكة توزيع طاقة صغيرة تحتوي على مولد

الشمسية ومصدر للطاقة

أقر بأن ما اشتملت عليه هذه الرسالة إنما هو نتاج جهدي الخاص، باستثناء ما تمت اإلشارة إليه حيثما ورد، وأن

ها لم يقدم من قبل االخرين لنيل درجة أو لقب علمي أو بحثي لدى أي مؤسسة هذه الرسالة ككل أو أي جزء من

تعليمية أو بحثية أخرى.

Declaration

I understand the nature of plagiarism, and I am aware of the Uuniversity's policy on

this. The work provided in this thesis, unless otherwise referenced, is the researcher's own

work, and has not been submitted by others elsewhere for any other degree or qualification.

:Student's name أحمد يوسف سكر اسم الطالب:

:Signature التوقيع:

:Date التاريخ:

IV

Abstract

Over the last ten years, Gaza Strip is suffering from inveterate crisis in the

electricity sector and since that time experts working in this area to find solutions

that contribute to alleviate the problem and commensurate with the available

resources. Therefore, Photovoltaic (PV) systems with power generator are used to

overcome this crisis. However, one of the biggest challenges of using PV sources

with electric generator is reverse power condition. When the volume of the load

become less than photovoltaic power systems, reverse power flow from the PV to

the generator may cause important problems if it is not considered in the protection

system design. One of the objective of this study is to investigate of the reverse

power condition of the hybrid power system including 5KVA diesel generator and

3KVA PV. This thesis proposes a power management device (PMD) in order to

solve the problem of reverse power flow from PV systems to the generator. The

proposed system consists of PIC18F microcontroller, relays, voltage and current

sensing component. The proposed power management device constructed at the

Islamic University of Gaza to serve the Electrical Engineering Laboratories. the

effectiveness of the PMD are evaluated by experimental analyses. The experimental

results showed that the amount of reverse power flow from PV power systems is

controlled at threshold of 10% of the diesel generator capacity. The result also

showed PMD were able prevent the reverse power flow from PV to protect the

other power sources.

Keywords: Photovoltaic, power distribution system, smart micro-grid

system.

V

الملخص

في دمج مصادر ومشهود لهاتقنية الشبكة الذكية لتوزيع الطاقة منتشرة في البالد المتحضرة

انتاجها. نظرا والمساهمة فيعلى ترشيد استهالك الطاقة المتجددة وتحفيز المواطنالطاقة

التي تقنن هذا الموضوع يعزف المستثمرين للطاقة ريعاتوغياب التشلتقطع التيار الكهربي

لألنظمةويتجهون ( On-Gridالشمسية عن بناء األنظمة المجهزة للتكامل مع الشبكة )

قلة تكلفتها وبالتالي( رغم ان األولى تمتاز بعدم احتياجها للبطاريات Off-Gridالمعزولة )

الذكية لدى صناع القرار في البلد وإدارة الشبكات . غياب الخبرة التقنية في بناءوصداقتها للبيئة

عن الشبكة االبتعادالقطاع الخاص في مجال الطاقة المتجددة يساهم سلبا في ولدي مستثمري

في بناء شبكة ذكية في هذه الرسالة ولهذا نقومغزة. الذكية كحل استراتيجي ألزمة الطاقة في

ويتم اي شبكة ذكية مستقبلية. والربط مع لتوسعوقابلة لكيلو وات 8خاصة بقدرة ابتدائية

النظام في الجامعة اإلسالمية ليخدم مختبرات قسم الهندسة الكهربائية بتغطية احتياجاتها تركيب

نحو الحلول والتدريس وتوجيه البوصلةاغراض البحث والمساهمة فيمن الطاقة

5مصدرين للطاقة مولد ديزل بقدرة ألزمة الطاقة. النظام المقترح يحتوي على االستراتيجية

يسي كيلو فولت امبير. الى جانب المصدر الرئ 3نظام خاليا شمسية بقدرة كيلو فولت امبير و

اإلدارة المنوي بناؤه يقوم بإدخال المصادر المتاحة حسب في حال توفره. نظام التحكم و

حماية علىوالعمل حمالاولويات األلويات المصادر واألحمال المطلوبة مراعيا التزامن واو

.مصادر الطاقة من األعطال الناتجة عن تشغيل النظام

VI

قال تعالي:

ويسألونك عن الروح قل الروح من أمر ربي وما أوتيتم من العلم إال

قليلا

صدق هللا العظيم

85] :ءاالسرا [

VII

Dedication

This thesis is firstly dedicated to God almighty and to my parents, who were

immense sources of support throughout my education, I thank you. Thanks for all

your love and support through the end.

VIII

Acknowledgment

I am deeply indebted to my Supervisor Prof. Dr. Mohamed M. Abdelati, for his

invaluable suggestions, help, and advice throughout this project. Thanks to Qatar

Charity for partially supporting this research under IBHATH project for research

grants, which is funded by the Cooperation Council for the Arab States of the Gulf

throughout Islamic Development Bank. I am also grateful to my colleagues at Mega

Power Company for their cooperation and technical support.

IX

Table of Contents

Declaration........................................................................................................................ II

Examiners Decision ........................................................................................................ III

Abstract ........................................................................................................................... IV

Abstract in Arabic ............................................................................................................. V

Epigraph Page ................................................................................................................. VI

Dedication ...................................................................................................................... VII

Acknowledgment .......................................................................................................... VIII

Table of Contents ........................................................................................................... IX

List of Figures ................................................................................................................. XI

List of Abbreviations ..................................................................................................... XII

Chapter 1 Introduction .................................................................................................. 1

1.1 Background ............................................................................................................. 2

1.2 Synchronous Generator Protection Review ............................................................ 5

1.3 Research Motivation ............................................................................................... 5

1.4 Research application ............................................................................................... 5

Chapter 2 PV Fundamental ........................................................................................... 7

2.1 Distributed Generation ........................................................................................... 8

2.2 Photovoltaic System ............................................................................................... 9

2.2.1 Electrical Characteristics of Photovoltaic Cells ........................................ 12

2.2.2 Advances in Photovoltaic Technology and Its Application ...................... 14

2.3 PV module and maximum power point tracking .................................................. 15

2.4 Converter and integration of PV to the grid ......................................................... 16

Chapter 3 Protective Relaying Fundamentals ........................................................... 17

3.1 Power system protection ....................................................................................... 18

3.2 Relaying ................................................................................................................ 18

3.2.1 Sensing Sources, Current and Voltage Transformers ................................. 19

3.2.2 Relay Characteristics .................................................................................. 19

3.2.3 Electromechanical to Electronic Relays ..................................................... 21

3.2.4 Advantages of Electronic Relays Over Electromechanical Relays ............ 21

3.2.5 Basic Construction of Electronic Relay...................................................... 22

3.3 Reverse Power Flow Mechanism ......................................................................... 22

3.4 Microprocessor Based Relays .............................................................................. 24

X

Chapter 4 System Design and Components ............................................................... 25

4.1 System Requirements ........................................................................................... 26

4.2 PV Source ............................................................................................................. 29

4.2.1 PV Panels .................................................................................................... 29

4.2.2 On-grid 3KVA Inverter .............................................................................. 29

Chapter 5 System Design and Experimental Results ............................................... 31

5.1 Voltage and Current sensors ................................................................................. 32

5.2 Hardware Design .................................................................................................. 35

5.3 Software Algorithm .............................................................................................. 37

5.4 Experimental results ............................................................................................. 40

Chapter 6 Conclusions and Future work ................................................................... 44

6.1 Conclusions .......................................................................................................... 45

6.2 Future work .......................................................................................................... 45

The Reference List ........................................................................................................ 46

Appendix 1 .................................................................................................................... 50

XI

List of Figures

Figure (1.1): Off-grid system. ...................................................................................... 3

Figure (1.2): On-grid system. ...................................................................................... 3

Figure (1.3): Circuit diagram of the main distribution board ...................................... 4

Figure (2.1): Annual solar PV Production. ................................................................ 11

Figure (2.2): I-V curve showing the maximum power point of PV. ......................... 13

Figure (2.3): Current-voltage characteristics. ............................................................ 14

Figure (3.1): Quantity vs. Time . ............................................................................... 20

Figure (3.2): Quantities vs. Phase Angle. .................................................................. 20

Figure (3.3): Forward power flow. ............................................................................ 23

Figure (3.4): Reverse power flow. ............................................................................. 23

Figure (4.1): Complete power and control circuit diagram. ...................................... 27

Figure (4.2): Complete power and control circuit board. .......................................... 28

Figure (4.3): 3KVA on-grid PV system. ................................................................... 29

Figure (4.4): Hybrid PV system. ............................................................................... 30

Figure (5.1): Voltage sensing. (a) AC input. (b) Conditioned signal. ....................... 33

Figure (5.2): AC current and voltage sensing circuit along with the power supply

unit.. ............................................................................................................................ 35

Figure (5.3): The microcontroller section of the device.. .......................................... 36

Figure (5.4): Signal processing for power calculation. ............................................. 39

Figure (5.5): The power management device.. .......................................................... 40

Figure (5.6): The power management device after connection... .................................. 41

Figure (5.7): Device testing first experiment.. .......................................................... 42

Figure (5.8): Second experiment.. ............................................................................. 43

XII

List of Abbreviations

A/B Analog-to-Binary

AC Alternating Current

CB Circuit Breaker

CG Contactor Generator

CM Contactor Main

CT Current Transformer

DC Direct Current

DERs Distributed Energy Resources

DG Distributed Generation

DPCA Distributed Power Coalition of America

DS Duty Cycle

ELCB Earth Leakage Circuit Breaker

GCB Generator Circuit Breaker

HPF High Pass Filter

KVA Kilo Volt Ampere

KWh Kilo Watt hour

MCB Miniature Circuit Breaker

MPPT Maximum Power Point Tracking

MTS Manual Transfer Switch

PV Photovoltaic

PWM Pulse Width Modulated

RMS Root Mean Square

RPR Reverse Power Relay

SCADA

STC

Supervisory Control and Data Acquisition

Standard Test Conditions

SWG Switch Generator

SWM Switch Main

VT Voltage Transformer

WTG wind Turbine Generators

1

Chapter 1

Introduction

2

Chapter 1

Introduction

1.1 Background

Micro-grid technology for energy distribution is widely used in developed countries.

It is recognized for integration of renewable energy sources and motivation of

citizens to rationalize energy consumption while contribution to energy production.

Any small scale electrical power generation technology that provides electric power

near a load site, with or without interconnection to any distribution system is known

as Distributed Energy Resources (DERs) or Distributed Generation (DG) (Bernal,

Dufo, Dom and Yusta, 2008). They can be operated according to customer’s demand

and facilities during peak hours as an auxiliary power source, thus emphasizing on

much greener sources of power supply. Of all the alternative resources, available,

photovoltaic system, wind turbines, synchronous generators driven by turbines and

fuel cells are amongst the most popular sources. Photovoltaic systems derive power

from the sun, which depends on many factors such as solar irradiation and light

intensity. These factors in turn affect the efficiency, voltage and output of the system

(Das, Esmaili, L. Xu, and Nichols, 2005).

Since most of these factors cannot be controlled individually, renewable energy

based DGs are usually composed of two or more systems to compensate one system

for other. In residential homes and small businesses facility people are used to store

electric power provided by the power distribution company by the use of batteries

and inverter system to use the stored energy during power outages. Although the use

of inverter system is considered better than electric generators, this solution involves

several disadvantages including changing the batteries periodically due to limited life

span. Also, due to the high price of the batteries and their large size, the energy that

can be stored is limited. Furthermore, the stored electric power can only be used in

small devices such as lighting and computers.

Over time the solar energy systems appeared on the market, this technology enabled

the user to run most of the loads during the day and store limited amounts of energy

3

in the system batteries to be used at night. This system is called (Off-Grid) as shown

in Figure 1.1.

Figure (1.1): off-grid system.

The disadvantage of this system is the high cost and also the manually split of loads.

In developed countries, they had developed less expensive and more efficient system

which is called (On-Grid) as shown in Figure 1.2.

Figure (1.2): on-grid system.

4

Figure (1.3): Circuit diagram of the main distribution board.

This system does not require batteries, but the users consume the demanded power

quantity and pump the overflowing energy to the distributer rather than storing it. Of

course, the main source of electricity is available around the 24 (Das et al 2005&

Zhu, D. Xu, PWu, Shen, and Chen, 2008). Such technology is unsuitable for Gaza

because of the frequent interruption (disconnect of the energy) of the main source of

power. Therefore, we strongly believe we need a combination of those sources as

shown in Figure 1.3.

5

1.2 Synchronous Generator Protection

Synchronous generators supply almost all the electric power we consume today. The

rotor of a synchronous generator needs to be driven by a source of mechanical power

or prime mover and the field winding needs to be fed by a source of DC power in

order to provide active and reactive power to the power system. These exclusive

combinations of electrical and mechanical equipment need to be protected against

different kinds of faults. Generator faults are always considered to be serious since

they can cause severe and costly damage to insulation, windings, core, shafts and

couplings. Protective relays using variety of signals are meant to monitor and provide

proper signals to alarm or remove the generator from the system under faulty

conditions. Such abnormal conditions and associated protective devices will be

discussed in chapter three.

1.3 Problem Statement

The objective this work is to use distributed energy sources consisting of

photovoltaic and synchronous generator. The use of renewable sources in

conjunction with the utility grid has some problems. One of the biggest problems

using renewable sources is its interconnection with the grid. Without any proper

control strategy to interconnect the individual systems together, the output power of

the distribution system cannot be regulated to meet the power demand. This would

also give rise to deficiency in the power quality and also stability problems in the

system. Thus, a proper control strategy is required to control one of the renewable

sources in order to meet the load demand while maintaining power quality.

Modelling a standalone system is a challenge, and connecting different renewable

sources together with the utility grid is the main focus of this research.

1.4 Research application

Distributed generators located at various places in the micro grid require protective

relays in order to change their protection and control settings. For example, when

distributed generators are placed at no identical place, power system improves

6

because of the resulting increased number of circuit paths to feed users in case of

equipment (utility source, breakers and/or power lines) failure or maintenance

operations. In certain circuit paths with distributed generators, currents along power

lines could flow in different directions depending on what type of distributed

generator was connected to end users the details regarding distributed generator are

explained in chapter two section 2.1.

7

Chapter 2

PV Fundamental

8

Chapter 2

PV Fundamental

2.1 Distributed Generation

Distributed Generation (DG) also known as decentralized generation is the future

trend of the present-day utility system due to its ability to accommodate various

alternative/ renewable energy sources. Distributed Generation will revolutionize the

world of power system as the internet did the world of computing. As the internet,

DG evolved from need for survivability with large mesh networks that would re-

route the outages and yet could sustain failures and maintain its stability. In the same

way, Distributed Generation, referred to as DG, is essentially known as the “Internet

of Energy,” by producing energy from very small generation sources, sustaining the

faults and yet supplying unlimited power (Ron and Wilder, 2007).

With the introduction of Distributed Generation, the energy sector has undergone

major changes in many countries. These changes created a very competitive network

driven by liberalization of market and decentralized power generation continues to

compete with the centralized generation market (Zobaa and Cecati, 2006). Different

countries use different definitions for DG to familiarize with the overall concept of

DG as discussed by (Zobaa and Cecati 2006). One such general definition suggested

is:

“Distributed generation is an electric power source connected directly to the

distribution network or on the customer side of the meter” (Ackerman et al, 2000).

Distributed Generation, when implemented, provides reliable, high quality, low-cost

electric power and offers savings in grid expansions and line losses. DGs, if

connected to the power grid, provide bidirectional power flow between the grid and

local generation unit, which enhances the total generation capacity, providing

uninterrupted power supply with optimum energy cost (Zobaa and Cecati 2006).

9

According to the Distributed Power Coalition of America (DPCA), DGs have the

potential to capture up to 20% of the entire power of the total generation capacity

(Borbely and Kreider, 2001). In today’s environment where issues such as blackouts

due to voltage instability, reliability and electricity costs are at its peak (Kurita and

Sakurai, 1988; US-Canada, 2003). Distributed Generation plays an important role.

The customers had to pay nearly double that of the usual price per kWh and such

kinds of power failures have become frequent in recent times. All these blackouts

clearly serve as a warning for the future scenario where Distributed Generation could

very well be a solution to all the above problems. From renewable to non-renewable

energy, Distributed Generation covers a wide spectrum of sources for electricity

generation. Distributed Generation should not be confused with renewable

generation; it may or may not be renewable sources. DG power sources also known

as Distributed Energy Resources (DERs) are of many types, namely photovoltaic

cells (PV), fuel cells, synchronous generators driven diesel engines, gas turbines or

micro turbines, wind turbine generators (WTG), and internal combustion engines to

name some of them. These DER capacities may range from 3kW to 10MW, which is

less than 500 MW capacity of centralized generation (Jin and Jimenez, 2010).

Distributed sources being green and environmental friendly, seldom contribute to air

pollution which is very minimal (Puttgen et al, 2003).

2.2 Photovoltaic System

The history of photovoltaic cells dates back to 1839 when a French physicist

Alexander Edmund Becquerel discovered that certain materials when exposed to

sunlight produced small electric currents. This discovery led towards an active

research interest for electricity generating materials, thus coining the term

“photovoltaic effect”. According to photovoltaic effect, when light falls on a

semiconductor material of the PV cell it absorbs light energy in the form of photons

which penetrates into the atoms of semiconductor.

10

This process frees the electrons in the negative layer to flow through an external

circuit and then back to the positive layer, thus producing a flow of electric current

(Williams, 1960). In 1957, Hoffman electronics produced solar cell with 9%

efficiency, the first radiation proof silicon solar cell was produced in the same year

too. On 17th March of the same year, the first satellite powered by solar cells,

Vanguard I, was launched (Perlin,1999). In 1963, Bell laboratories in the USA

launched the first commercial telecommunications satellite, Telstar. The usage of

solar cells in this expedition has brought a revolutionary interest in application of

solar cells in space programs. This major event even marked a great interest in use of

solar cells as a major source of power supply in communication and satellite

industry. After Telstar space expedition, the solar cells were in demand due to their

efficiency. (Figure 2.1) clearly shows the world-wide increase in the production of

PV power in megawatts, annual solar photovoltaics module production in china from

2007-2016. The demand started depreciating as the production cost per watt was

higher when compared to fossil fuels.

Even though solar cell’ efficiencies were increasing, its cost per watt was still too

high when compared to fossil fuels, thus making it only viable to use in places

connected to small utility grids and lines. Lately the production costs of solar cells

are declining rapidly making these available for use in simple daily products like

calculators and watches. This has also pushed solar cells to be economically more

viable to be used not only in standalone applications but also with power system.

11

annual solar photovoltaics module capacity in china from 2007-2016.

Figure (2.1): Annual solar PV Capacity

The arrangement of photovoltaic cells in arrays is a notable process. A set of

modules are aligned together to form arrays. These arrays are known as solar panels

and are prominently used in households. A few important points about PV arrays:

To increase the utility and efficiency, PV cells are interconnected together

using a sealed, waterproof package to form “Modules”. When two modules

are interconnected together in series, their voltage doubles and the current

stays constant.

When two modules are wired together in parallel, the current is doubled

keeping the voltage constant and giving maximum power. To get the

maximum desired voltage, modules are wired together in series and parallel,

to form PV arrays. This arrangement can produce power anywhere in the

presence of direct sunlight.

12

2.2.1 Electrical Characteristics of Photovoltaic Cells

Electrical characteristics of PV cells are defined as the behaviour of cell under

different conditions, that are best determined using the current and voltage levels at

different irradiance levels or when connected with different loads. When the PV cell

has, no load connected to it, the voltage across it reaches maximum. This is known as

“open circuit voltage.” When a load is connected across the PV cell, the current

reaches its maximum and the voltage decreases. This current is referred to as “short

circuit current.” A nonlinear relation exists between the PV cell's current and voltage.

This relation indicates that there is a variation in voltage and current of PV cell with

any change in the load. Figure 2.2 shows the I-V curve of a PV cell, this I-V curve is

used to determine the characteristics of the PV cell used in solar modules. At

Standard Test Conditions (STC), comparisons between performances of different PV

cells are made. STC is defined as the light intensity at 1000 W/m2 of solar irradiation

and 25ºC temperature of PV cell. The Pmax on the figure below represents the

maximum power of the PV; at this point the power (product of output voltage and

current of PV) reaches its maximum point. It is also known as “Maximum power

point”. The solar cell has a maximum output current called short circuit current (ISC),

open circuit voltage (VOC), and a maximum output voltage. PV modules usually

supply power in peak watts (WP) measured at STC.

13

Figure (2.2): I-V curve showing the maximum power point of PV

The characteristics and efficiency of a PV cell depends on many factors such as the

size and material, irradiance, temperature, and light intensity. The current is directly

proportional to light intensity, unlike voltage which varies slightly by changing light

intensity. Figure 2.3 shows the change in current voltage curve under changing

irradiation levels at 25ºC and light intensity at 1000 W/m2 (Brea, 2009); the short

circuit current is significantly affected by change in irradiation.

14

Figure (2.3): Current voltage characteristics

Figure 2.3 Current voltage characteristics of PV the performance of the Photovoltaic

cell also depend upon the temperature. Voltage mainly is affected by change in

temperature. An increase in temperature decreases the voltage and the current

increases by small amounts.

2.2.2 Advances in Photovoltaic Technology and Its Application

Photovoltaic energy has been substantially beneficial due to less air pollution and

zero emissions into the environment. One disadvantage with PV is its high cost of

installation. But, with the ongoing improvements in PV technology and from the

experiences gained by installing thousands of systems, PV energy and its

performance have increased by leaps and bounds (Barker and Bing, 2005). The costs

of PV cells have also dropped in the past few decades, thus making it more

accessible to customers (Akihiro, 2005). Other than the space satellites and

household applications, photovoltaic systems are being used in rural areas, such as in

rural health clinics for refrigeration, water pumps for irrigation, and small scale

15

power generations (Zobaa and Cecati, 2006). PV sources have become popular for

off-grid and on-grid market, making it more economical (Barker and Bing, 2005).

The output power of the PV system depends on many variable factors such as the

solar insolation levels, light intensity and temperatures. This is one drawback of a

photovoltaic generation system, as the insolation level is not always constant to

produce the expected energy and lowers the reliability of the generation system

(Billinton, 2001; Liang et al, 2001). Many methods are developed to locate and

obtain the maximum power from the cells (Kim el al, 2001).

The photovoltaic system can be connected to the grid which can act as pool of energy

from which the power can be taken and also given back depending upon the

insolation level and local energy demand (Zobaa and Cecati, 2006). Maximum

Power Point Technique (MPPT) is one such technique used to extract the maximum

power from the PV cells at any given point of solar insulation. The output of PV is

DC and it must be converted to AC using DC/AC converter and this AC output can

be interfaced with the utility grid of large or small scale. On the other hand, super

capacitors or batteries can be used to store energy for standalone PV systems

(Jamehbozorg, et al, 2011).

2.3 PV module and maximum power point tracking.

A photovoltaic module consists of interconnected solar cells to form a single unit. A

non-linear relation exists between voltage and current of a solar cell, shown in Figure

2.2, are plotted by simulating the simple PV module/equivalent circuit of

photovoltaic module (Durgadevi, et al, 2011). The output of photovoltaic cell

depends on the solar insolation at specific levels; at a particular operating point the

output of the PV system is maximum. This maximum efficiency helps in extracting

the maximum power. It is difficult to achieve an optimum insolation value. This

maximum output can be extracted using the Maximum power point tracking

(MPPT). MPPT is used in many applications in connection with PV system like in

PV water pumps systems (Yoo, 2010) and low cost household applications (Brea,

16

2007). Hence, it is desirable to obtain maximum power point output at minimum cost

under various operating conditions. Due to the importance of MPPT, several studies

have been published (Esram and Chapman, 2007; Faranda, and Maugeri, 2008)

comparing the different methods of working, namely, (i) Perturb and Observe, (ii)

incremental conductance, (iii) constant voltage, and (iv) Fuzzy logic control.

2.4 Converter and integration of PV to the grid.

A conventional two stage conversion unit is generally required for a photovoltaic

system to be interfaced with electric power system. The first stage would be the

DC/DC converter, which in turn is to track the maximum power point from the PV

system. The DC output from the MPPT is converted to AC output such that the

voltage level of the photovoltaic system is adjusted to the grid level. This is the

second stage which involves the DC/AC converter (Durgadevi, et al, 2011). This

converter is controlled to produce the output voltage to match with the grid quality

during standalone mode. During the grid connected mode the output current is

matched in phase with the electric grid such that power is transferred to the grid at

unity power factor (Zobaa and Cecati, 2006). The controllers are designed based on

control goals and depending upon the DC/DC and DC/AC converters. The efficiency

of the conversion system depends on various factors and individual devices

connected like converters, batteries, and switches (Liang, Kuo and Chen, 2001). The

DC to AC conversion efficiency of modern day converters has proven to exceed

90%. The most recent converters use the maximum power point technology for

extracting the power from the photovoltaic system by dynamically adjusting the DC

bus voltage of the PV system to its optimum value of changing insolation level and

temperature.

17

Chapter3

Protective Relaying

Fundamentals

18

Chapter3

Protective Relaying Fundamentals

3.1 Power system protection

The primary purpose of power system protection is to ensure safe operation of power

systems. Furthermore, the tasks are to minimize the impact of unavoidable faults in

the system most protective relays use digital processors to deploy a wide range of

functions, and are programmable to allow more sophisticated criteria for initiating

interrupt procedures to be applied. Modern digital relays have also started to fulfill a

monitoring function since they can record the voltages and currents they measure for

a period before and after any fault (Power System Relaying Committee, 2007).

Power plant generators are important components of an electrical energy system,

they should be constantly monitored and protected in order to maintain the quality

and reliability of the power supply, otherwise, generators may occur in case of faults

or incorrect operation. One of them is reverse power condition, reverse power flow

causes problems if it is not considered in the protection system design. One of the

objective of this study is to investigate the reverse power condition of the generators

for this purpose, reverse power data are collected from cogeneration power plant

generators (Power System Relaying Committee, 2007).

3.2 Relaying

The principal task of a protection system is to detect abnormal conditions, including

faults in power system equipment such as generators, transformers, transmission

lines, substations, and distribution systems. In doing so, the protection system should

also initiate action to limit to a minimum the damage caused by the fault or abnormal

condition and shorten, as far as possible, the duration of the power supply

interruption. Immediate isolation of the fault not only restricts damage to the

equipment, but also contributes to maintaining the stability of the system.

The isolation of a fault and protection of the equipment is usually the responsibility

of a protection scheme based on relaying devices. The designers consider normal

19

systems operations and expected malfunctions, but base their protection designs

mostly on the worst possible scenario. A protective relay is a sensing device which,

when energized by suitable currents, voltages, or both, responds to the magnitudes

and relationships of those currents and voltages by sending (relaying) a signal which

will activate appropriate isolation equipment.

3.2.1 Sensing Sources, Current and Voltage Transformers

Since the protective relays operate from currents and voltages supplied by current

transformers (CTs) and voltage transformers (VTs), their operative function cannot

be any better than their associated transducers. (Current and potential transformers

do not "sense" in the strict use of the term. They serve to transform currents and

voltages accurately to convenient amplitudes and to provide electrical isolation). It is

therefore necessary that the outputs of the instrument transformers present a realistic

picture to the relays of the conditions in the primary circuit during faults, as well as

during normal operations.

3.2.2 Relay Characteristics

A relay may be activated by a single quantity, such as a current or voltage, or by two

quantities (same or mixed). In the latter case the relay may be made to respond to the

phase angle between the two quantities, or to the relative magnitude of the two

quantities or to a combination of the magnitude and the phase angle. The relation

between the quantities that will cause the relay to operate may be shown graphically

by what is called the operating characteristic. When the relay is activated by a single

quantity, its response is purely a function of time as in Figure 3.1. When the relay is

activated by two quantities, the characteristics may be shown in terms of the

magnitude of one quantity and the phase angle between the two quantities as in

Figure 3.2.

20

Figure (3.1): Quantity vs. Time

Figure (3.2): Quantities vs. Phase Angle

21

3.2.3 Electromechanical to Electronic Relays

Over seventy percent of the existing protective systems in our present-day power

systems are based on electromechanical relays. The schemes in use depend on the

characteristics of induction disc or cup, moving coil or moving ferro-magnetic

element often termed as armature elements. Several issues and considerations have

influenced the pace of development in electronic relays:

1. Better performance and characteristics, e.g., higher speed with greater

accuracy and sensitivity.

2. Greater standardization in manufacturing.

3. Easier manufacturing and reduction in maintenance time. Initially, electronic

relays were designed using thermionic tubes, but the drawbacks of providing

special power supplies for cathode heating and the provision of special

voltages for anodes and electrode bias led researchers to solid state

components, and recently to the microprocessor.

3.2.4 Advantages of Electronic Relays Over Electromechanical Relays

Electronic relays in general possess the following advantages:

1. Low burden on current and voltage transformers when the power is operated.

2. Absence of mechanical inertia and burning contacts; high resistance to shock

and vibration.

3. Very fast operation and long life.

4. Low maintenance owing to the absence of moving parts and bearing friction.

5. Quick reset action and absence of overshoot.

6. Ease of providing high amplification factors to provide increased sensitivity.

7. A greater degree of selectivity in the shaping of operations characteristics,

enabling the practical utilization of relays with operating characteristics more

closely approaching the ideal requirements.

8. Miniaturization of the relay modules due to low energy requirements.

22

The electronic relays with solid state components have certain limitations, but these

can be overcome as indicated.

1. Temperature: sensitivity temperature compensation circuits have been

developed and new solid state devices are less sensitive to temperature

variations.

2. Aging: this is eliminated by presoaking of components for several hours at a

relatively high temperature.

3. Sensitivity to voltage spikes: this can be reduced by filters and shielding.

4. Damage due to overloading: this can be reduced by careful circuit design.

5. Limited reliability: this can be reduced by utilizing redundant circuits and

logic.

Electronic relays may be single or multiple-input devices. Individual modules are

now on the market to provide critical measuring as well as non-critical switching

functions. Timing and connection requirements are much better satisfied by solid-

state circuits than with electromagnetic elements.

3.2.5 Basic Construction of Electronic Relay

Basically, electronic protective relays based on current or voltage comparison are

Analog to binary (A/D) signal converters with square wave output which can be

supplied to a comparator circuit in such a way that a triggering signal is obtained

only when both the observed and comparison signals are present the duration of the

loop or cycle time is based on a timer circuit which can be adjusted accordingly with

the type of application.

3.3 Reverse Power Flow Mechanism

Radial distribution networks are usually designed for unidirectional power flow,

from the in feed downstream to the loads. This assumption is reflected in standard

protection schemes with directional over current relays. With a generator on the

23

distribution feeder, the load flow situation may be changed. The generator used is a

synchronous machine which can either run as generator or motor depending upon the

form of input energy. The definition of motor and generator operation relates to the

sign of the average power. During the motoring action of the generator the power

flows from the bus-bars to the machine. This condition is called as reverse power

flow (Contino, Iannone, Leva, and Zani, 2006).

Figure (3.3): Forward power flow

Figure (3.4): Reverse power flow

24

3.4 Microprocessor Based Relays

A microprocessor or a computer is inherently programmable and has basically simple

logic and large memory capability, because whenever it strikes a limit another

processor can be added to extend it further. Also, computer technology has the

inherent capability for data transfer, mathematical or logic analysis, communication

between modules both local and remote, data compression and reporting any

exception. The capability of "reporting by exception" is ideal for relaying functions

as during normal power system operation the microprocessor is gathering and

"processing" the data collected, but it is the data during the instability period, when it

is needed most and a tripping decision is made, which is reported under this rule of

exception capability and stored in the memory.

These capabilities make adaptive functions much more feasible with the

microprocessor technology. That is, the protection control and monitoring functions

can automatically adjust their performance to match the ever-changing requirements

of a dynamic power system and can also handle and optimize the changing system

configuration. With the trend setting breakthroughs in the microprocessor based

technology of the past decade, the implementation of more elaborate protection

schemes with adaptive features are becoming practical. The use of these adaptive

features in combination with the microprocessor's self-diagnostic capability, make it

possible to further enhance protective schemes for the substations. A trend is

developing to make it a part of a general Supervisory Control and Data Acquisition

System (SCADA), but still capable of making a tripping decision at a module level

for the sake of information and cost savings on hardware involved.

25

Chapter 4

System Design and

Components

26

Chapter 4

System Design and Components

This chapter introduces the design of the system and experiments by using the main

power sources. It gives a general understanding of how to design experiments. Refer

to subsequent chapters in this thesis.

4.1 System Requirements

The system was constructed at the Islamic university of Gaza to serve the Electrical

Engineering Laboratories. It will help in meeting its energy needs while being

valuable tool for teaching and research purposes. Moreover, it will help attracting

attention to the strategic solutions of the energy crisis. The system includes two

sources of power. 5KVA diesel generator and a 3KVA PV system along with the

main power supply. It is enough resources of power to run the renewable laboratory

in our department and test the proposed device. The control and management system

helps synchronizing the On-Grid inverter with the diesel generator while keeping the

generator loaded above a specified threshold (about 20%). In this study, we plan to

analyse all possible scenarios and configure the controller to act properly.

The aim of the system is to help interconnect a different of power resources as well

as reduction of the power crisis in Gaza. Therefore, the components used to build this

system were selected according to available component in local markets. However,

the complete system shown in Figure 4.1 and Figure 4.2.

27

Figure (4.1): complete power and control circuit diagram.

28

Figure (4.2): complete power and control circuit board.

29

4.2 PV Source

The use of PV source of operate with on-grid technology with 3 KVA capacity as

shown in figure 4.3.

Figure (4.3): 3KVA on-grid PV system.

4.2.1 PV panels

Canadian Solar cell new modules have significantly raised the standard of module

efficiency in the solar industry. They introduced innovative four bus-bar cell

technology, which demonstrates higher power output and higher system reliability.

4.2.2 on-grid 3KVA inverter

We used 3KVA on-grid InfiniSolar inverter, this hybrid PV inverter can provide

power to connected loads by utilizing PV power, utility power and battery power as

shown in figure 4.4.

30

Figure (4.4): Hybrid PV system.

Depending on different power situations, this hybrid inverter is designed to generate

continuous power from PV solar modules (solar panels), battery, and the utility.

When MPP input voltage of PV modules is within acceptable range, this inverter is

able to generate power to feed the grid (utility) and charge battery. This inverter is

only compatible with PV module types of single crystalline and poly crystalline.

31

Chapter 5

System Design and

Experimental Results

32

Chapter 5

System Design and Experimental Results

5.1 Voltage and Current sensors

The voltage signal will be fed to one of the analog input channels of the

microcontroller. The range of the analog input is from 0 to 5 volts for the targeted

PIC18F microcontroller family. Therefore, a signal conditioning circuit is necessary

to match the line voltage whose nominal root mean square (rms) value is 220V to fit

within the allowable analog input range of the microcontroller as illustrated in Figure

4.1 Utilizing the full dynamic range of the analog channel helps in achieving the best

possible resolution of the analog to digital conversion process. To this end, the AC

voltage signal will be converted to have a peak to peak value of 5V (1.768 V rms)

and then shifted 2.5V. Conversion could be done by a simple voltage divider.

However, this approach leads to common ground between the microcontroller circuit

and the line input which has many safety and disturbance concerns. Therefore, a

transformer will be utilized insuring galvanic isolation between the controller circuit

and the line voltage. A standard transformer 220V to 12x2V rated 300 mA is

modified to have an additional winding which produce a maximum of 1.768 V rms

for the maximum expected line voltage which is about 240V rms. The original

secondary windings of this transformer will be used to generate a 12V along with a

regulated 5V power sources necessary for the controller circuit.

33

(a)

(b)

Figure (5.1): Voltage sensing. (a) AC input. (b) Conditioned signal

As the number of primary winding is unknown to us, the required number of turns of

the voltage sensing windings is determent experimentally and found to be 37. Now

the primary number of turns may be calculated for future use as

𝑁𝑝=37×240÷1.768=5023. The resulted signal is clamped over a 2.5V DC so that the

conditioned signal always lay within the analog input range of the microcontroller

which is 0-5 volts.

34

For AC, current sensing, there are many techniques used in the industry. One may

use a small series resistor and measure the voltage drop across it. This method is not

prober here as it does not provide galvanic isolation. Another method is to use Hall

Effect current sensor integrated circuits such as Allegro ACS712. Such chip senses

the magnetic field generated by the current flowing through the sensors legs, which

in turn outputs an analog voltage proportional to the current it senses. While being,

efficient and have small footprint, they are unavailable in the local market

unfortunately. The last common method is to use the current transformer (CT)

technique. Although not available in the sieged Gaza, we found a way to build it

locally using a standard 220V to 12x2V 300 mA transformers. The low voltage

winding has been totally removed and replaced by 2 turn winding of an insulated

copper cable whose cross-sectional area is 4 mm2.

This 2-turn winding is now the primary winding of the current transformer. Using a

low value burden resistor across the 5023 turn secondary winding, one may estimate

the primary current by measuring the voltage drop on the burden resistor. In order to

calculate the value of the burden resistor, the maximum current that will be measured

should be specified. In our case this value is 20A rms. The turn’s ratio of the CT is 2

to 5023. Therefore, the maximum current in the secondary winding is

𝐼𝑠 = 𝐼𝑝𝑁𝑝

𝑁𝑠= 8 mA (1)

To maximize measurement resolution, the voltage across the burden resistor at peak-

current (8√2 mA) should be equal to one-half of the analog reference voltage (5V).

Therefore, the ideal burden resistor is

𝑅 =2.5×103

8√2= 221 Ω

(2)

In our implementation, we used the nearest standard resistor which is220 Ω±1%. The

voltage across the burden resistor is clamped over a 2.5 V DC to keep the signal

within the analog input range of the microcontroller. Figure 3 shows the complete

35

circuit used for voltage and current sensing along with the power supply unit.

Resistors R1 and R2 form a voltage divider to obtain the required 2.5 V DC shift

source and the diodes D1 to D4are used to clip any possible spikes that may damage

the controller. The relay (RL1) allows the controller to disconnect the generator from

the grid in case of faults. The solid-state relay will be excited by a pulse width

modulated (PWM) control signal to operate a burden load that keeps the generator

load above a specified threshold.

Figure (5.2): AC current and voltage sensing circuit along with the power supply

unit.

5.2 Hardware Design

The device is required to read the instantaneous voltage and current signals and use

them to compute and display the standard electrical measurements such as rms

voltage, rms current, power, and power factor. The device should be able to keep the

power above a preset value through controlling a burden load. Moreover, it should be

36

able to disconnect the generator from the grid in case of any fault. The device is

better featured with communication channels for possible interconnection with other

devices in the grid such as the power inverter or a grid management controller. In

addition, these channels may facilitate uploading the expected device firmware

updates. To meet these requirements, a PIC18F2550 microcontroller is selected. It is

ideal for low power and connectivity applications that benefit from the availability of

a USB port and an asynchronous serial port (EUSART). Has 2 pulse width

modulation (PWM) outputs, rich of analog and digital channels, possess large

amounts of RAM memory for buffering and Enhanced FLASH program memory

make it suitable for embedded control and monitoring applications. The circuit

diagram of the controller unit is shown in Figure 5.3.

Figure (5.3): the microcontroller section of the device.

Along with the microcontroller chip, a Max232 integrated circuit is added to convert

signals from a RS-232 serial port to the TTL signals of the microcontroller’s USART

serial port. The USB port of the microcontroller is ready to use after connecting the

37

200 nF capacitor at pin 14. The switch SW1 allows the device to be powered through

the host USB port. This option is valuable during software developing and firmware

uploading.

5.3 Software Algorithm

The voltage wave of the generator output is sinusoidal with frequency 𝑓 and peak

value of 𝑉𝑝. It may be expressed as.

𝑣𝑙(𝑡) = 𝑉𝑝 cos(2𝜋𝑓𝑡) (3)

The voltage signal at the analog to digital converter input (𝑉𝑎(𝑡)) is a shifted scaled

version of the line voltage. The DC shift is half the analog to digital reference

voltage 𝑉𝑟 and the scaling factor (𝑘𝑣) is the voltage transformer turns ratio. For our

specific case 𝑉𝑟 is 5V and 𝑘𝑣 is 37/5023. Therefore, the signal at the analog to digital

converter input is expressed as follows:

𝑣𝑎(𝑡) = 𝑘𝑣𝑉𝑝 cos(2𝜋𝑓𝑡) +𝑉𝑟

2 (4)

The analog to digital converter has a resolution of 10 bit (1024 levels). Assuming a

sampling period of 𝑇𝑠 and neglecting the quantization error, the nth digital samples

will equal:

𝑣𝑑(𝑛𝑇𝑠) =2𝑚−1

𝑉𝑟(𝑘𝑣𝑉𝑝 cos(2𝜋𝑓𝑛𝑇𝑠) +

𝑉𝑟

2) (5)

The scaling number comes from the fact that when the analog input equals the

reference value, it is converted to the maximum 10-bit digital number which

is 2𝑚 − 1. In order to obtain a digital value equals the line voltage along with some

DC offset that will be removed later; the converted signal must be multiplied by the

scaling factor𝑉𝑟

𝑘𝑣(2𝑚−1). The resultant signal will equal:

38

𝑣𝑠(𝑛𝑇𝑠) = 𝑉𝑝 cos(2𝜋𝑓𝑛𝑇𝑠) +𝑉𝑟

2𝑘𝑣 (6)

Using a High Pass Filter (HPF) the DC content of the signal is eliminated producing

a clean digital version of the line voltage signal:

𝑣(𝑛𝑇𝑠) = 𝑉𝑝 cos(2𝜋𝑓𝑛𝑇𝑠) (7)

The line current waveform will be also sinusoidal having the same frequency but

with peak value of 𝐼𝑝 and a phase shift θ that depend on the applied load. The

current may be expressed as:

𝐼𝑙(𝑡) = 𝐼𝑝 cos(2𝜋𝑓𝑡 + 𝜃) (8)

Using the same procedure carried for the voltage, we will obtain a clean digital

version of the line current signal which equals:

The extracted voltage and current signals are multiplied to give the instantaneous

power signal:

𝑃(𝑛𝑇𝑠) = 𝑉𝑝𝐼𝑝 cos(2𝜋𝑓𝑛𝑇𝑠) cos(2𝜋𝑓𝑛𝑇𝑠 + 𝜃)

= 𝑉𝑝𝐼𝑝 cos(2𝜋𝑓𝑛𝑇𝑠) [cos 𝜃 cos(2𝜋𝑓𝑛𝑇𝑠) − sin 𝜃 sin(2𝜋𝑓𝑛𝑇𝑠)]

= 𝑉𝑝𝐼𝑝 cos 𝜃 cos2(2𝜋𝑓𝑛𝑇𝑠) −1

2𝑉𝑝𝐼𝑝 sin 𝜃 sin(4𝜋𝑓𝑛𝑇𝑠)

=1

2𝑉𝑝𝐼𝑝 cos 𝜃 [1 + cos(4𝜋𝑓𝑛𝑇𝑠)] −

1

2𝑉𝑝𝐼𝑝 sin 𝜃 sin(4𝜋𝑓𝑛𝑇𝑠)

=1

2𝑉𝑝𝐼𝑝 cos 𝜃 +

1

2𝑉𝑝𝐼𝑝[cos 𝜃 cos(4𝜋𝑓𝑛𝑇𝑠) − sin 𝜃 sin(4𝜋𝑓𝑛𝑇𝑠)]

𝐼(𝑛𝑇𝑠) = 𝐼𝑝 cos(2𝜋𝑓𝑛𝑇𝑠 + 𝜃) (9)

39

=1

2𝑉𝑝𝐼𝑝 cos 𝜃 +

1

2𝑉𝑝𝐼𝑝cos(4𝜋𝑓𝑛𝑇𝑠 + 𝜃) (10)

Substituting 𝑉𝑝 = √2𝑉rms and 𝐼𝑝 = √2𝐼rms into previews equation, the instantaneous

power formula becomes:

𝑃(𝑛𝑇𝑠) = 𝑉𝑟𝑚𝑠𝐼𝑟𝑚𝑠 𝑐𝑜𝑠 𝜃 + 𝑉𝑟𝑚𝑠𝐼𝑟𝑚𝑠𝑐𝑜𝑠(4𝜋𝑓𝑛𝑇𝑠 + 𝜃) (11)

The first term is constant and known as real or active power while the other term is

oscillating between the source and load. It is known as the reactive power. In order

to extract the active power component, the instantaneous power is filtered with a low

pass filter (LPF). A block diagram that illustrates the signal processing for power

calculation is shown in Figure 5.4.

Figure (5.4): Signal processing for power calculation.

For the control process the calculated power is compared to a preset threshold (about

20% of the generator rating) the error signal is used to set the duty cycle of the PWM

control signal that activates the burden load. The duty cycle may range from 0 to

255. If not reached its maximum and the power is still less than the threshold, the

40

duty cycle is increased quickly. On the other hand, if not reached its minimum and

the power is above the threshold, the duty cycle is decreased slowly until it reaches

10% after which the decrement is performed in a much slower rate. Defining a long

integer variable DSL that is 100 times the duty cycle DS, the proposed algorithm is

easily implemented using integer arithmetic as follows:

5.4 Experimental results

The device is implemented on an 11x13 cm printed circuit board as illustrated in

Figure 5.5. The Mikroelektronika USB boot loader is downloaded on the

microcontroller using a chip programmer for once. Later, the USB port is used to

download the controller software. We started be simple software routines to test the

hardware functionality such as the display, the relay, the solid-state relay, and the line

measurements. Doing this successfully, the controller code is installed and the device

is connected with the inverter as illustrated in Figure 5.6.

Figure (5.5): The power management device.

41

Figure (5.6): The power management device after connection.

For the sake of the generator safety, we preferred to test the device first with the

mains supply.

42

Figure (5.7): Device testing first experiment.

The power threshold is set to 300W and the experiment is conducted at noon time

when the inverter was able to inject up to 2400 W into the grid. A variable load is

used while running connection and disconnection of the inverter and the mains

circuit breakers. All scenarios are successfully conducted and the controller was able

to keep the power drown from the grid positive.

The second experiment was to test the device for generator protection. Therefore, it’s

integrated with our experimental platform illustrated in Figure 1.3. The first run of

the experiment did not go as expected. The generator we are using is small size rated

5 KVA. There was a significant tolerance of its frequency especially on load

variation. Under no load its frequency was 51.6 Hz and this lays outside the

43

allowable frequency range for the inverter which is from 47.5 Hz to 51.5 Hz. Once

loaded its frequency drops to about 49 Hz. To fix the situation we preferred to adjust

the generator governor so that it has a frequency of 50.5 Hz at no load. Moreover, the

power threshold is raised to 500 W instead of 300 W. It is about 10% of the

generator rating and secures a wide margin for transients. Having done these

modifications, the system operated properly without any complications.

Figure (5.8): Second experiment

44

Chapter 6

Conclusion and Future

work

45

Chapter 6

Conclusion and Future work

6.1 conclusion

When connecting an on-grid PV system with a power generator, it is expected to

share the load and help reducing the consumption of generator fuel. However, if the

load is lower than the injected PV power, energy will flow in the generator disturbing

its operation and risking its safety. The main objective of this research was to design

and implement a device that regulates such a situation. The device is based on

PIC18F2550 microcontroller and programmed to keep the generator running above

10% of its rating. The device is tested successfully on an experimental platform at

the Islamic university of Gaza.

The implemented device will enable institutions which has backup generator to

integrate renewable systems in their power grid. However, a complementary research

is expected to integrate this device with a higher-level controller for the institution.

6.2 Future work

Its recommended to perform this type of work with wider power demand and

building whether in IUG or other organization.

Other researcher may proceed to perform an automatic control to connect and

disconnect between the power resources.

Another future work suggested to use a telecommunication and mobile technology to

control the load type that will be easier to the users.

46

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47

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